The creamed coconut is a sort of almost-solid, almost waxy in texture white stuff that comes in a little plastic sachet and melts into the curry to make it taste really silky and subtly coconut-y. Mmmm...

And yes, I can see that sweet potato would work just as well, given that it has the same sort of sweetness and texture. I'm using butternut squash because we grew it this year for the first time (in among the corn, a bit of Native American companion planting there!) and for almost no effort whatsoever got some fantastic squashes. It'll be on our growing list again next year :-)